Concurrent Programming Actors, SALSA, Coordination Abstractions

1. Concurrent ProgrammingActors, SALSA, Coordination Abstractions Carlos Varela RPI C. Varela

2. Overview of concurrent programming • There are four basic approaches: • Sequential programming (no concurrency) • Declarative concurrency (streams in a functional language) • Message passing with active objects (Erlang, SALSA) • Atomic actions on shared state (Java) • The atomic action approach is the most difficult, yet it is the one you will probably be most exposed to! • But, if you have the choice, which approach to use? • Use the simplest approach that does the job: sequential if that is ok, else declarative concurrency if there is no observable nondeterminism, else message passing if you can get away with it. C. Varela

3. Actors/SALSA • Actor Model • A reasoning framework to model concurrent computations • Programming abstractions for distributed open systems G. Agha, Actors: A Model of Concurrent Computation in Distributed Systems. MIT Press, 1986. • SALSA • Simple Actor Language System and Architecture • An actor-oriented language for mobile and internet computing • Programming abstractions for internet-based concurrency, distribution, mobility, and coordination C. Varela and G. Agha, “Programming dynamically reconfigurable open systems with SALSA”, ACM SIGPLAN Notices, OOPSLA 2001, 36(12), pp 20-34. C. Varela

4. SALSA Support for Actors • Programmers define behaviors for actors. • Messages are sent asynchronously. • State is modeled as encapsulated objects/primitive types. • Messages are modeled as potential method invocations. • Continuation primitives are used for coordination. C. Varela

5. Actor Creation • To create an actor locally TravelAgent a = new TravelAgent(); • To create an actor with a specified UAN and UAL: TravelAgent a = new TravelAgent() at (uan, ual); • At current location with a UAN: TravelAgent a = new TravelAgent() at (uan); C. Varela

6. Message Sending TravelAgent a = new TravelAgent(); a <- book( flight ); C. Varela

7. Remote Message Sending • Obtain a remote actor reference by name. TravelAgent a = getReferenceByName(“uan://myhost/ta”); a <- printItinerary(); C. Varela

8. Migration • Obtaining a remote actor reference and migrating the actor. TravelAgent a = getReferenceByName(“uan://myhost/ta”); a <- migrate( “rmsp://yourhost/travel” ) @ a <- printItinerary(); C. Varela

9. Token Passing Continuations • Ensures that each message in the continuation expression is sent after the previous message has been processed. It also enables the use of a message handler return value as an argument for a later message (through the token keyword). • Example: a1 <- m1() @ a2 <- m2( token ); Send m1 to a1 asking a1 to forward the result of processing m1 to a2 (as the argument of message m2). C. Varela

10. Join Blocks • Provide a mechanism for synchronizing the processing of a set of messages. • Set of results is sent along as a token. • Example: Actor[] actors = { searcher0, searcher1, searcher2, searcher3 }; join { actors <- find( phrase ) } @ resultActor <- output( token ); Send the find( phrase ) message to each actor in actors[] then after all have completed send the result to resultActor as the argument of an output( … ) message. C. Varela

11. Example: Acknowledged Multicast join{ a1 <- m1(); a2 <- m2; a3 <- m3(); … } @ cust <- n(token); C. Varela

12. Java Foundry SALSA Acknowledged Multicast 168 100 31 Lines of Code Comparison C. Varela

13. First Class Continuations • Enable actors to delegate computation to a third party independently of the processing context. • For example: int m(…){ b <- n(…) @ currentContinuation; } Ask (delegate) actor b to respond to this message m on behalf of current actor (self) by processing its own message n. C. Varela

14. Fibonacci Example module examples.fibonacci; behavior Fibonacci { int n; Fibonacci(int n) { this.n = n; } int add(int x, int y) { return x + y; } int compute() { if (n == 0) return 0; else if (n <= 2) return 1; else { Fibonacci fib1 = new Fibonacci(n-1); Fibonacci fib2 = new Fibonacci(n-2); token x = fib1<-compute(); token y = fib2<-compute(); add(x,y) @ currentContinuation; } } void act(String args[]) { n = Integer.parseInt(args[0]); compute() @ standardOutput<-println(token); } } C. Varela

15. SALSA and Java • SALSA source files are compiled into Java source files before being compiled into Java byte code. • SALSA programs may take full advantage of the Java API. C. Varela

16. Hello World Example module demo; behavior HelloWorld { void act( String[] args ) { standardOutput<-print( "Hello" ) @ standardOutput<-println( "World!" ); } } C. Varela

17. Hello World Example • The act( String[] args ) message handler is similar to the main(…) method in Java and is used to bootstrap SALSA programs. • When a SALSA program is executed, an actor of the given behavior is created and an act(args) message is sent to this actor with any given command-line arguments. C. Varela

18. Migration Example behavior Migrate {void print() { standardOutput<-println( "Migrate actor is here." ); }void act( String[] args ) {if (args.length != 3) {standardOutput<-println("Usage: salsa Migrate <UAN> <srcUAL> <destUAL>"); return; } UAN uan = new UAN(args[0]); UAL ual = new UAL(args[1]); Migrate migrateActor = new Migrate() at (uan, ual); migrateActor<-print() @ migrateActor<-migrate( args[2] ) @ migrateActor<-print(); }} C. Varela

19. Migration Example • The program must be given valid universal actor name and locations. • After remotely creating the actor. It sends the print message to itself before migrating to the second theater and sending the message again. C. Varela

20. Compilation • Compile Migrate.salsa file into Migrate.java. • Compile Migrate.java file into Migrate.class. • Execute Migrate $ java SalsaCompiler Migrate.salsa SALSA Compiler Version 1.0: Reading from file demo/Migrate.salsa . . . SALSA Compiler Version 1.0: SALSA program parsed successfully. SALSA Compiler Version 1.0: SALSA program compiled successfully. $ javac Migrate.java $ java Migrate $ Usage: java Migrate <uan> <ual> <ual> C. Varela

21. theater 2 theater 1 Migration Example UAN Server The actor will print "Migrate actor just migrated here." at theater 1 then at theater 2. C. Varela

22. Host Location OS/JVM Processor yangtze.cs.uiuc.edu Urbana IL, USA Solaris 2.5.1 JDK 1.1.6 Ultra 2 vulcain.ecoledoc.lip6.fr Paris, France Linux 2.2.5 JDK 1.2pre2 Pentium II 350Mhz solar.isr.co.jp Tokyo, Japan Solaris 2.6 JDK 1.1.6 Sparc 20 Local actor creation 386us Local message sending LAN message sending WAN message sending 148 us 30-60 ms 2-3 s LAN minimal actor migration LAN 100Kb actor migration WAN minimal actor migration WAN 100Kb actor migration 150-160 ms 240-250 ms 3-7 s 25-30 s World Migrating Agent Example C. Varela

23. Exercises • How would you implement the join continuation linguistic abstraction in terms of message passing? • Download and execute the Migrate.salsa example. • Write a solution to the Flavius Josephus problem in SALSA. A description of the problem is at VRH Section7.8.3. C. Varela